It is desirable to produce light olefins (e.g., ethylene, propylene, and butenes) by cracking relatively heavy hydrocarbon feedstocks, such as gas-oils and crudes, utilizing pyrolysis or steam cracking. It is also required that the cracked effluent stream is quenched or cooled shortly after leaving the pyrolysis furnace to prevent the cracking reactions from continuing past the point of product generation. Quenching effluent streams from cracked heavy hydrocarbon feed presents special challenges to prevent deposition of tar (including tar-precursors and other heavy components) and related fouling problems within the quench equipment. Further, it is desirable to improve steam cracking process efficiency by indirect heat exchange and reuse of heat recovered from the cracked effluent stream. Effluent heat recovery is typically performed by indirect heat exchange, such as with one or more transfer line exchangers (TLE's).
Hydrocarbon feed is heated rapidly during cracking, typically in the presence of steam. After heating and cracking, the vaporized effluent stream may typically exit the pyrolysis furnace at high temperature, such as from about 785° C. (1450° F.) to about 930° C. (1700° F.) and must be rapidly quenched to halt the cracking reactions and prevent degradation of the valuable products. In addition to producing olefins, steam cracking heavier hydrocarbon feedstocks, including feedstocks having aromatic components associated therewith, also produces reactive molecules that tend to combine or polymerize with each other while hot to form higher molecular weight materials known as tar, pitch, or non-volatiles (referred to collectively herein, as tar). Tar is a relatively high-boiling point, viscous, active material that, under certain conditions can deposit on, insulate, plug, and foul heat exchange equipment. The fouling propensity can be characterized in three temperature regimes.
At temperatures above the dew point (the temperature at which the first drop of liquid condenses) of the cracked furnace effluent, the fouling tendency is relatively low. Vapor phase fouling is generally not an issue as there is no liquid or condensates present that could cause fouling or polymerize. Appropriately designed transfer line heat exchangers operating in this regime may quench and remove heat with minimal fouling by limiting the amount of cooling affected to maintain the effluent in the vapor phase.
Below the stream dew point, steam cracked tar condenses from the effluent stream and the fouling tendency may be relatively high, particularly at and immediately downstream of the location where the dew-point is reached. In some applications, as additional materials subsequently condense, there may be sufficient low-viscosity liquids present to flux or carry away the high molecular weight tar molecules. In this regime, the heaviest components in the stream condense but remain hot enough to remain reactive and sustain dehydrogenation and polymerization reactions, undesirably forming higher molecular weight tar molecules. The tar condensates tend to adhere to inner surfaces of process equipment, such as in the TLE's. Furthermore, this material adheres to surfaces and continues to polymerize, dehydrogenate, thermally degrade, and harden, thus making it difficult to remove.
At or below the temperature at which tar is fully condensed, the fouling tendency is relatively low, due to depressed thermal activity and due to the presence of sufficient condensates to act as solvent to keep the tar flowing in the liquid phase. In this regime, the condensed material is still hot enough and fluid enough to flow readily at the conditions of the process but fouling is generally not a serious problem. Phase separation and fractionation becomes key objectives at this stage, to separate the tar and liquids from the more valuable vaporized effluent that comprises the olefin products.
In view of condensation-related fouling and equipment build-up, cracked gas oil and cracked heavy hydrocarbon effluent streams, including some cracked naphtha effluent streams, cannot easily be cooled directly to a desirable processing temperature range, such as from 230° C. to about 300° C. (450° to 570° F.), due to the presence of the condensable tar components. To mitigate tar deposition and prevent fouling, it is known to provide quench fluid injection for direct introduction of a cooling direct quench fluid, directly into the hot effluent stream and/or on the effluent through bore. Direct quench is commonly performed by introduction of the direct quench fluid into the effluent through bore, typically onto both the effluent through bore wall and within the effluent stream, and is dispersed through gravity, fluid shear, and/or mechanical dispersion during introduction. Direct quench is also commonly conducted by dispersing the direct quench fluid directly onto the bore wall. A direct quench cooling process primarily cools by direct mixing and contact of the direct quench fluid with the effluent, such that the direct quench fluid absorbs heat from the hot effluent and may additionally include quench fluid evaporation, both from the bore wall and from within the stream flow path. As the effluent cools, some components therein may condense and replace a portion of the vaporized quench fluid. This direct quench process serves primarily to reduce the temperature by heat transfer to and by at least partial evaporation of the quench fluid. If sufficient volume of quench fluid is introduced, some of the fluid may remain in the liquid phase, depending of course upon the final boiling point of the direct quench fluid, and the direct quench fluid may act as a carrier for the condensed components and simultaneously coat/wet the inner surface of the quench exchanger with quench liquid and thereby prevent accumulation of fouling tar, coke, and precipitates on equipment surfaces.
Significant drawbacks to such direct-quench systems are the high required direct quench fluid injection volume and the corresponding high separation and treatment volumes and costs. It is common for such systems to introduce in excess of three to four mass units of quench fluid per mass unit of process effluent. Pipe sizing must be increased to accommodate such volumes. On commercial sized crackers, this can result in undesirably large circulation pumps, pipe work, cost, and energy consumption. Further, due to the difficulty in controlling the physical dispersion of the injected quench fluid within the cracked effluent stream and equipment process surfaces, not only are large amounts of quench fluid used, but the introduction systems also may utilize inertial dispersion, spraying, or some other type of voluminous and energetic introduction method to attempt adequate dispersion and mixing to directly quench the cracked effluent stream. An additional and serious operation problem with dispersion fittings is the propensity of the small openings in the nozzles to plug with polymer and coke particles.
Separate from direct fluid quench, another means of quenching hot effluent is with an indirect heat exchanger, such as a TLE, either with or without concurrent direct quench injection, though typically without express creation of a wetted-wall quench fluid film. The art has desired production of a wetted wall indirect heat exchanger quench process but has had difficulty actually achieving a commercially effective and efficient process or apparatus. Whereas with the previously discussed direct quench apparatus, a wetted wall film may contribute at least partially to quenching the effluent stream, the role of a wetted wall quench film in an indirect heat exchange apparatus is primarily to mitigate fouling, while merely acting as a medium to transfer heat from the effluent stream to the indirect cooling medium in a cooling jacket that is exterior to the effluent conduit. In an indirect heat exchange process, the coolest region is close to the bore walls and as such, foulants tend to accumulate on the cool walls. The wet surface film is desired to act primarily as an impediment to foulant deposition and as a carrier for removal of condensates and tar precursors from the system, which might form either due to condensation within the effluent stream, or from effluent proximity to the relatively cooled effluent bore wall. The difficulty, however, has been in affecting comprehensive heat exchanger wall film coverage over the full circumference and length of the exchanger in the presence of a shearing, hot, gaseous effluent flow. Not only has the problem been difficult to achieve, it has been even more difficult to do so efficiently. The known indirect heat exchange quench systems that attempt to utilize a wetted wall process are inefficient and commercially deficient for the intended purpose, requiring introduction of undesirably excessive amounts of quench fluid.
The article “Latest Developments in Transfer Line Exchanger Design for Ethylene Plants”, H. Herrmann & W. Burghardt, Schmidt'sche Heissdampf-Gesellschaft, prepared for presentation at AIChE Spring National Meeting, Atlanta, April 1994, Paper #23c, discloses dew point fouling mechanisms in ethylene furnace quench systems, as well as use of heat exchangers that generate high pressure steam, e.g., a quench exchanger followed by a quench fluid injection fitting. However, need for process and equipment improvements remain.
U.S. Pat. Nos. 4,107,226; 3,593,968; 3,907,661; 3,647,907; 4,444,697; 3,959,420; 4,121,908; and 6,626,424; and Great Britain Patent Application 1,233,795 disclose various dry wall, sequential dry wall, and direct quench, and quench fluid direct injection fittings, applications, including annular introduction fittings. These references also disclose various methods of distributing wash liquids in annular quench fittings. U.S. Pat. No. 3,593,968 discloses a method and apparatus for direct oil quench point, with no heat recovery to another medium. Also, under actual operating conditions and manufacturing variations, the severe temperature differences of the various components, heat stresses, and repeated heating and cooling cycles create difficulties in creating and maintaining a uniform film coverage and thickness. These deficiencies resulted in utilization of excessive amounts of quench fluid to maintain operational effectiveness. Other attempted improvements followed in the art. In U.S. Pat. No. 3,959,420, the same inventor provided an improved annular quench fitting that reversed the position of some of the quench fluid discharge components as compared to the '968 patent, providing a method and apparatus similar to a spill-over or weir apparatus to control flow of the quench fluid. The operational effectiveness of such design tends to be subject to equipment alignment and manufacturing variances and also requires excessive quench fluid flow rates to overcome the deficiencies. The '420 design also requires additional components and complexity, such as a baffle and introduction of an inert gas in a purge gas chamber. Differential movement and distortion between the abutting sections of the injector can adversely affect the quench oil injection pattern and is not effective for quench to feed mass ratios of less than about 2.0. Further improvements continued to be sought in the industry.
U.S. Pat. No. 4,121,908 teaches use of tangential introduction of liquid quench fluid in attempt to utilize inertial energy to disperse the direct quench fluid circumferentially on all surfaces of the quench bore. Again however, this process also requires use of an inefficiently large quantity of combined quench fluid, as the liquid quench fluid is introduced into the bore along with a direct quench fluid into the same bore that conveys the gaseous effluent. Further, the apparatus of the '908 patent possesses areas along the quench tube bore that are subject to fouling tar build-up, including the tube areas opposite the locations of introduction of the liquid quench fluid. The apparatus of the '908 invention also cannot produce a uniform liquid quench film at the desired low quench fluid rates or ratios.
U.S. Pat. No. 4,444,697 discloses a direct quench fitting and teaches use of tangential introduction of direct quench fluid directly into the effluent through bore, using multiple openings in an attempt to provide full quench fluid film coverage and concurrent dissipation for direct quenching. However, the tangential quench oil distribution and introduction is performed in an annular cavity that performs both distribution within the cavity and direct introduction into the through bore. The arrangement directs a substantial portion of quench fluid immediately into the effluent through bore from the slots nearest each point of introduction of quench fluid into the cavity. There is insufficient hydraulic control of the introduced quench fluid. To distribute quench fluid to other slots requires introduction of an inefficient volume of quench fluid and disproportionate distribution of quench fluid on the bore circumference. The annular, multiple introduction slot arrangement fails to adequately control distribution of quench fluid about the full length of the annular cavity, by permitting excessive introduction nearest the quench fluid source with dissipating rates through the length of the annular cavity. Also, as with many of the preceding designs, the tangential quench fluid introduction ports are also inefficiently designed, creating discontinuous fluid introduction into the bore, leading to areas of foulant formation. Further, the fluid inlet ports are positioned to direct quench fluid directly at a few of the inlet slots, further contributing to inefficient performance. Still further improvements were needed.
U.S. Pat. No. 6,626,424 discloses a method for quenching a hot effluent stream by injecting a quenching fluid tangentially, directly into the hot gas stream with sufficient inertia and momentum to cause the quench fluid to flow circumferentially around the inside surface of the conduit. However, quench fluid introduction systems such as disclosed in the '424 patent and others listed above that introduce the quench fluid directly into the effluent conduit from a single point or from a discrete number of points require an inefficient volume of quench fluid. Also, computer modeling has demonstrated that separated phase flow patterns or regimes tend to establish along the flow path as the volume of quench fluid is reduced to desirably efficient levels, requiring use of an inefficient volume of fluid to obtain suitable surface coverage over the full length of the TLE. Also, quench introduction fittings tend to be sized to operate around a target flow range and if the effluent flow diverges out of this flow range, then the fitting is either inefficiently over-sized or under-sized. To avoid these issues, such systems tend to require introduction of an excessive volume of quench fluid to overcome the non-uniformity and dispersional inefficiencies. Further, a significant portion of the quench fluid is introduced in such manner as to directly and transversely encounter the high velocity cracked effluent stream, resulting in turbulent dispersion within the flow stream and mitigated interaction with tube process surfaces. This tends to result in substantial portions of the introduced quench fluid inefficiently not encountering and not protecting the inner process wall. To mitigate the turbulent dispersion effect, an excessive volume of quench fluid is introduced to improve surface coverage efficiency. Again, this also requires increased processing equipment capacity.
The prior art demonstrates that the processes and apparatus for introducing a wall-wetting quench fluid via the known quench fittings and processes have efficiency shortcomings and often produce less than optimal quench results. The prior art leaves room for further process and equipment improvements to achieve the desired operational efficiency and effectiveness in a quench system for quenching a tar-bearing cracked effluent while mitigating tar buildup on the process surfaces of the quench tube.
It remains desirable to provide an improved quench fluid introduction method and apparatus that more efficiently, uniformly, and conservatively distributes an efficient amount of quench fluid along the effluent through bore. It is desirable to provide a wet wall quench system that is useful with a direct quench system and/or an indirect heat exchange system, that also effectively uses substantially less quench fluid than prior art systems to prevent tar buildup. Further, it is desirable to reduce the amount of quench fluid required to effectively coat the quench apparatus effluent through bore surfaces. It is desired to provide an effective, comprehensive, wetted wall quench fluid film that uses less quench fluid than is required by the prior art processes.